944 EVOLUTIONAL MODIFICATIONS 



may be termed, is a characteristic of the primates. In effect it is the con- 

 sequence of expansion taking place in the neopallium which begins to over- 

 shadow the archipallium (rhinencephalon). The chief increments are at 

 first evident in the parietal lobe, indicating no doubt the expansions in som- 

 esthetic sensibility due to quadrumanal differentiation. Corresponding 

 expansion soon makes itself apparent in the temporal and occipital lobes. 

 It finally expresses itself, at the upper extremity of the primate series, in 

 large increments to the frontal lobe. This dimensional modification is expres- 

 sive of a recession in the rhinencephalon, particuhirly involving the olfactory 

 sense which is gradually replaced in the jjrimate by an extensive develop- 

 ment in the distance receptors for sight and hearing. In certain other groups 

 of mammals there is a marked degree of broad brainedness, for example, 

 that seen in seals, cetaceans or sirenians. In these instances the brains are 

 almost as broad as they are long, and thus transcend the upper limit already 

 prescribed by the diametric indices of the primate brain. In the case of 

 aquatic mammals, the underlying motive in the transverse expansion of 

 the hemispheres is different from that in the primates. It particularly 

 affects the parietal area in relation to expanding the region of somesthetic 

 sensibility. These animals, by their entire body surface, are in constant 

 contact with the water. Their somatic sensory surface thus becomes a most 

 important receptor system and because of its great extensiveness determines 

 the corresponding expansion of the parietal lobe. Some of the acjuatic 

 mammals also show a recession in the pruniinenee of the rhinencephalon 

 and the olfactory sense, which may even exceed that witnessed in apes and 

 man. They show, however, no such expansion in the frontal or occipital regions 

 as that seen in the primates. 



The Fissural Pattern of the 1 Iemispiiekes in the Primate Brain. 

 The lissural pattern of the In'misplieres is identical with or lollows a de- 

 sign similar to the liunuin adult. Fhe eon\e\ity oi the cerebral lurui- 



